In the past few years, surface plasmon resonance (SPR) sensors have become a standard analytical tool for various applications such as life sciences applications, pharmaceutical applications, thin-film metrology and bio-chemical sensors. An SPR sensor typically includes a glass prism having a thin metallic layer deposited on a prism face. The active interface (i.e., sensing interface) of the SPR sensor is the exposed surface of the metallic layer.
In operation, light is totally internally reflected within the prism from the metal-coated surface of the prism, which causes an evanescent optical wave to penetrate into the metallic layer. This evanescent wave can couple to and excite a propagating surface plasmon wave at the sensing interface. Such coupling to the surface plasmon wave can be observed as a reduction in the internally reflected beam power, since the power required to excite the surface plasmon wave comes from this beam. The efficiency with which power is transferred between the incident beam and the surface plasmon wave depends on how nearly a resonance condition is satisfied (i.e., the amount of detuning from resonance). For the on-resonance condition (i.e., detuning=0), power transfer is maximal, and as the magnitude of the detuning increases, power transfer decreases.
In practice, this resonance can be probed by monitoring reflected power as the incident beam angle is varied, or as the incident wavelength is varied, since varying either of these parameters can alter the detuning. In either case, a significant dip in reflectance is observed when passing through the resonance. The particular wavelength (or incident angle) at which resonance occurs is a sensitive function of conditions at the sensing interface, since these conditions influence the properties of the surface plasmon wave. In particular, the resonance is a sensitive function of the refractive index of an analyte in contact with the sensing interface on the metallic layer. As an alternative to measuring the resonant angle (or wavelength, the reflectance can be monitored for a fixed angle of incidence and wavelength as the analyte varies. Typical sensitivities for conventional SPR sensors are on the order of 10−5 to 10−6 refractive index units (RIU).
As a first approximation, total internal reflection is a specular reflection process that behaves as expected from geometrical optics. However, non-specular effects in total reflection are known to occur, although these effects tend to be quite small and are often regarded as negligible in practice. One such non-specular effect is the Goos-Hänchen (GH) effect, which is a lateral spatial shift of the reflected beam away from the position expected from geometrical considerations. This effect was first directly measured in 1947 in a difficult experiment where a beam shift on the order of an optical wavelength per reflection was directly measured in the pre-laser era. Multiple reflections were performed in order to increase the observed effect. Indirect measurements of quantities related to the GH effect (such as reflective phase shifts) are sometimes also referred to as measurements of the Goos-Hänchen effect, although this usage can be misleading because such measurements are typically much easier than direct GH measurements. For example, Hashimoto et al. in an article entitled “Optical heterodyne sensor using the Goos-Hänchen shift” (Opt. Lett. 14(17) 913-915 1989) consider a sensor using multiple total reflections that is sensitive to the difference in reflective phase shift between s and p polarized light. Here also, multiple total reflections are performed in order to increase the measured signal.
Although the GH effect typically provides a lateral shift on the order of an optical wavelength, there have been demonstrations of an enhanced GH effect in certain circumstances. An enhanced GH effect has been observed in reflection or total reflection from structures having a surface plasmon resonance. For example, Bonnet et al. investigate large positive and negative GH shifts from metallic gratings in an article entitled “Measurement of positive and negative Goos-Hanchen effects for metallic gratings near Wood anomalies” (Opt. Lett. 26(10) 666-668 2001). An SPR enhanced GH effect in a prism TIR geometry is considered by Abbate et al. in an article entitled “Observation of lateral displacement of an optical beam enhanced by surface plasmon excitation” (J. Mod. Opt. 35(7) 1257-1262 1988). In these articles, lateral shifts on the order of 10-100 λ are observed.
Various approaches have been considered for improving the sensitivity of SPR sensors. For example, the use of long range surface plasmons is considered by Nenninger et al. in an article entitled “Long-range surface plasmons for high-resolution surface plasmon resonance sensors” (Sensors and Actuators B 74 145-151 2001). Long range surface plasmons are coupled surface plasmon waves propagating on opposite interfaces of a thin metallic layer. Other approaches that have been considered include performing differential measurements, fabricating resonant structures such as nano-particles, and the use of phase-sensitive techniques such as ellipsometry and interferometric detection. Of these approaches, phase sensitive techniques appear to provide the best sensitivity, although such techniques also and undesirably introduce significant additional complexity into the sensor.
Accordingly, it would be an advance in the art to provide an SPR sensor having improved sensitivity and/or reduced complexity compared to conventional SPR sensors.